1. Field of the Invention
The present invention relates to a technique for controlling the power output of an amplifier, in particular that of a Cartesian amplifier without causing distortion to the wanted signals i.e. the signals being amplified. The power control is applied by varying the forward and reverse path gains of the Cartesian loop.
A description of the operation of a linearised amplifier based on the Cartesian loop is given in our U.S. Pat. No. 5,381,108 entitled "Automatic Calibration of the Quadrature Balance within a Cartesian Amplifier". This shows how the loop uses modulation feedback to reduce the unwanted intermodulation products in the RF output.
Power control can provide significant advantage in a mobile radio environment (to which Cartesian amplifiers have found application). By reducing the transmit power to the minimum for good communications, co-channel and adjacent channel interference can be reduced and battery life in hand portable equipment can be extended. Thus a method that allows efficient power control for a Cartesian amplifier without interfering with the wanted signal or the performance of the loop is highly desirable.
Conventional power control can be applied in two ways. Firstly, a power attenuator can be used on the output of the amplifier, but this is wasteful of both power and hardware. Secondly, direct control of the amplifier output can be used by, for instance, controlling the supply voltage fed to it or by controlling any bias current. Controlling the amplifier output is of little use in the case of a Cartesian amplifier as the loop will always compensate for a reduction in amplifier gain by increasing the drive level. This would result in the amplifier being overdriven, causing a significant increase in distortion.
In the case of a linear amplifier, such as results from the application of a Cartesian loop, power control can be applied by reducing the drive level at the input to the amplifier system. This requires that the dynamic range of the modulating signal be extended to include the power control range. A number of factors limit the dynamic range of a Cartesian loop; noise and pick-up effects and, more importantly, the unwanted carrier output from the amplifier. This second effect is due to DC offsets in the baseband circuitry within the loop. Any offsets appearing at the output of the differential amplifier will result in carrier appearing at RF. Because the carrier is in the centre of the RF output frequency spectrum, it will interfere with the wanted signal if it is too large. Methods do exist for reducing DC offsets, but achieving very low levels for long periods of time is difficult and the residual carrier output limits the dynamic range of the Cartesian amplifier.
The transfer function of a feedback amplifier is of the general form ##EQU1## Where K and H are the forward and reverse path transfer functions respectively and KH represents the loop gain.
It is evident that power control which is inversely proportional to the level of some power control signal can be achieved by varying the gain in the feedback path.
However this approach means that the loop gain varies in direct proportion to the gain in the reverse path. Thus the loop must be designed to be stable at the highest level of loop gain which occurs at the lowest power output. It must also be designed with sufficient loop gain at the highest power output to reduce any unwanted non linear distortions from the power amplifier.
Thus, compared with an amplifier without power control, this simple form of power control requires the use of a more linear power amplifier for any given loop configuration and required output linearity.
As the drive level to a Cartesian loop is altered, the transistor in the power amplifier will be driven to different portions of its characteristic operating curve. This will result in a variation of its average gain with drive level. Any resulting change in the forward gain can effect the power control methods described above.
In the approach described above, no attempt is made to compensate the loop gain as power control is applied. This means that no compensation for the response of the power amplifier can be made. Typically the gain of the amplifier will reduce as the signal level increases and the transistor approaches saturation. In the other direction the loop gain will be increased by the power amplifier at low output power levels. This tends to extend further the stability margin required to operate power control in this way.